Unit 7 of 11 1 hr learning time

Making a Choice

A program that decides. Test a value, and do one thing if the test passes and another if it doesn't — the moment a program can react, and the start of an actual game.

64% of General Programming

Mark as complete Completed!

Everything so far runs the same way every time, whatever the values are. The program can't yet react. This unit changes that: it teaches the computer to decide — to do something only if a test passes. That single idea is the difference between a program that recites and a program you can play.

Do it only if

A decision is a test with a true-or-false answer, and an instruction to carry out only if the answer is true. In pseudocode:

ASK "Guess my number (1-10)? " INTO guess
IF guess = 7 THEN SHOW "Correct!"

In BASIC:

  10 INPUT "Guess my number (1-10)? "; g
  20 IF g = 7 THEN PRINT "Correct!"

Line 20 asks a true-or-false question — is g equal to 7? — and only runs the PRINT if the answer is true. Guess 7:

A Spectrum screen showing: Correct! — Guessed `7`, the test `g = 7` was true, so the `PRINT` ran. Guess anything else and the test is false — the `PRINT` is skipped, and nothing is shown.

That's a condition: a question with only two possible answers, true or false. When it's true, the instruction after THEN happens; when it's false, it's skipped. (Note that = here is asking "are these equal?" — a question — not "put into the box" from Unit 6. Same symbol, two jobs; the computer tells them apart by where it sits.)

React differently

One test is a yes/no. Real choices need more than that — too low, too high, spot on. The tests < (less than) and > (greater than) give us the rest:

Add hints for a guess that misses

1	1		`10 INPUT "Guess my number (1-10)? "; g`
2	2		`20 IF g = 7 THEN PRINT "Correct!"`
	3	+	`30 IF g < 7 THEN PRINT "Too low"`
	4	+	`40 IF g > 7 THEN PRINT "Too high"`
3	5

Now every guess is met by exactly one of three reactions. Guess 4:

A Spectrum screen showing: Too low. — `4` is less than `7`, so `g < 7` was true and `g = 7`, `g > 7` were false. Of the three tests, one passed — and you got a hint instead of a verdict.

Three lines, three tests, and only the one that's true does anything. That is genuinely a tiny game: a secret, a guess, and a response that depends on the guess. It just can't let you try again yet — that's the next idea but one.

Conditions can also be combined. g > 0 AND g < 11 is true only when both halves are (a guess in range); g = 1 OR g = 7 is true when either is. AND demands both; OR accepts either — the two ways of joining yes/no questions into a bigger one.

When it's wrong, see why

The reaction never happens. The test is never true — check it against the value you typed. If you guess 7 and nothing says "Correct!", read line 20: is it testing g = 7, and is g the box you put the guess in?
Two reactions happen at once. Two of your tests can be true together. g < 7 and g < 10 are both true for a 4. Make the tests cover separate cases — <, =, > share nothing.
It always reacts, whatever you type. Look for a test that's always true — g > 0 for a guess that's never negative, say. A condition that can't be false isn't a decision.

What you've learnt

A condition is a test with a true-or-false answer; an IF carries out its instruction only when the test is true, and skips it otherwise.
The tests are = (equal), < (less than), > (greater than) and their kin — and = as a question is a different job from = as put-into-a-box.
Conditions combine with AND (both must hold) and OR (either will do).
A decision is what lets a program react — the first thing that makes it feel like a game, not a recital.

What's next

The guessing game can tell you too high or too low — but then it stops, and you have to run it again to guess once more. A real game would let you keep going. In Unit 8 we meet the first kind of loop: doing something a set number of times without writing it out over and over.